This invention relates generally to the semiconductor art, and more specifically is concerned with a process for manufacturing solar cells using semiconductor material.
With the increasing cost and potential restrictions on availability of conventional fuels, e.g., petroleum, natural gas, etc., as sources of energy, considerable research effort has been directed towards developing other sources of energy. Notable among alternative sources of energy is the sun. In attempting to make use of the sun's energy, a substantial amount of research effort has been directed toward developing inexpensive, large-capacity devices referred to as solar cells which directly transform the sun's energy which initially is in the form of photons into electricity.
A solar cell typically includes a layer of material having a photovoltaic capability, wherein electric charges present in the material are freed as a consequence of light impinging on the material. The freed electrical charges, when separated from the region of their generation, produce a current flow, which may be routed to an outside circuit and into an electrical load where work is performed.
Since the current produced by the solar cell continues only so long as light actually impinges on the cell, those applications which have continuous current demands require a separate subsystem for electrical storage. In other applications, however, continuous current is not necessary, and in those applications, the solar cell suffices by itself.
The photovotaic material used in solar cells must have two fundamental characteristics: (1) an inherent structural capability of producing mobile charge carriers in response to adsorption of light; and (2) an internal potential barrier by which the mobile charge carriers freed by the light can be separated from the region in which they are generated. Generally, most semiconductor materials fulfill the first requirement, and conventional PN junctions, as found in solid state diodes and transistors, fulfill the second requirement. In addition to these characteristics, the photovoltaic material used in a solar cell as a practical matter possess numerous other properties and characteristics which contribute to the efficiency of the device. Detailed information of the desirable characteristics of such materials for use in solar cells may be found in numerous standard texts such as Solid State Physical Electronics, by Aldert Vander Ziel, published by Prentice-Hall, Inc., 1957.
Generally, numerous materials and compounds are suitable for use in solar cells, among them being silicon, selenium, gallium arsenide, cadmium telluride, and copper sulfide, to specify but a few of the more prevalently used materials and compounds. Considerations of expense and efficiency indicate that the semiconductor material should be either single-crystal or large-grain polycrystalline rather than small-grain polycrystalline, and should be inexpensive and readily available. Silicon is such a material although even those solar cells which currently use silicon are too expensive to complete economically with other sources of electricity.
There are several reasons why solar cells using silicon are still too expensive to manufacture economically on a large scale. First, solar cells heretofore have been made in unit sizes by a batch process, instead of discrete sizes by a continuous process. Second, the solar cells now produced are relatively small, instead of large surface area configurations. Attempts to data to produce large area solar cells have resulted in poor quality and extremely fragile structures, which are easily damaged in use. Furthermore, severe manufacturing problems have been encountered in such attempts.
Third, the cost of producing large area thin-film single-cell silicon arrays, by the current method of slicing and polishing blocks of single crystal silicon is too expensive for commercial use. Previous attempts to deposit silicon in thin films of semiconductor quality suitable for use in solar cells have been unsuccessful. This lack of success is due to many factors, among the most important being (1) the deposited silicon acquires significant amounts of impurities in the manufacturing process, thereby substantially reducing the efficiency of the resulting solar cell, and (2) the grain size of the deposited silicon is too small to produce the necessary conversion efficiency for practical use.
These and other disadvantages have combined to detract from the attractiveness of the silicon solar cell as a significant alternative to present sources of electricity, even though the source of energy utilized by the solar cell is virtually inexhaustable and readily available.
Accordingly, it is one object of the present invention to provide a process for producing solar cells and the solar cell produced thereby which overcomes one or more of the disadvantages of the prior art specifically discussed above.
It is another object of the present invention to provide such a process which produces solar cells in large-area configurations.
It is a further object of the present invention to provide such a process which is adapted for continuous, rather than batch, process application.
It is yet another object of the present invention to provide such a process and product which minimizes the use of semiconductor material in the solar cell product.
It is a further object of the present invention to provide such a process and product wherein the semiconductor material may be deposited in small-grain polycrystalline form.
It is another object of the present invention to provide such a process wherein the deposited polycrystalline silicon is modified into large-grain or single-crystal silicon, thereby improving conversion efficiency of the solar cell product.
It is a still further object of the present invention to provide such a process and product wherein impurities are prevented from entering the semi-conductor material during the process.
It is another object of the present invention to provide such a process and product wherein the solar cell product is economically competitive with other sources of electricity.